EP 722850B1 discloses a low-permeability thermoplastic elastomer composition that is superior as a gas-barrier layer in pneumatic tires. This thermoplastic elastomer composition comprises a low-permeability rubber, such as brominated poly(isobutylene-co-paramethylstyrene), referred to hereinafter as BIMS, dispersed in a low-permeability thermoplastic matrix, such as polyamide or a blend of polyamides. In EP 857761A1 and EP 969039A1, the viscosity ratio of the thermoplastic matrix and the dispersed rubber phase was specified both as a function of the volume fraction ratio and, independently, to be close to a value of one to produce a high concentration of small particle size vulcanized rubber particles dispersed in a thermoplastic phase. EP 969039A1 further disclosed that small particle size rubber dispersed in a thermoplastic resin matrix is important to achieve acceptable durability of the resulting composition, particularly where used as innerliners in pneumatic tires.
Compositions exhibiting low gas permeability performance (i.e., functioning as a gas barrier) composed of thermoplastic resin/thermoplastic resin-based blends such as a high density polyethylene resin and nylon 6 or nylon 66 (HDPE/PA6.66), a polyethylene terephthalate and aromatic nylon (PET/MXD6), a polyethylene terephthalate and vinyl alcohol-ethylene copolymer (PET/EVOH), where one thermoplastic resin is layered over the other layer to form plural layers by molding, and processes for producing the same, have been disclosed. An application regarding the use of such a composition as the innerliner layer of a tire is disclosed in Japanese Patent Application No. 7-55929. However, since these materials are thermoplastic resin/thermoplastic resin blends, while they are superior in gas barrier performance, they lack flexibility, and therefore, such films are subject to failure if they are used in a vehicle tire which is subject to significant stress and flexing.
Further, there are also examples of the use of a thermoplastic elastomer composed of a rubber and a thermoplastic resin for use as an innerliner or in a tire; see, Japanese Patent Application No. 8-183683, but in general, a flexible material of the type disclosed therein and having superior durability has low heat resistance. With a thermoplastic elastomer using a thermoplastic resin having a melting point less than the tire vulcanization temperature as a matrix, when the tire vulcanization bladder is released at the end of the tire vulcanization cycle, the tire inside surface is subject to appearance defects due to the thermoplastic resin sticking to or rubbing against the vulcanization bladder.
In preparing tire innerliners for assembly with a tire carcass, a thermoplastic elastomer is co-extruded together with an adhesive layer in two layers in cylindrical shapes. If the adhesive layer arranged at the outside is too tacky, it will stick to the take-up roll at the time of shaping and seriously degrade the workability. Further, this requires the insertion of release paper, Mylar® film, etc., between the tubular materials at the time of take-up, which is a problem in terms of cost and transport weight. On the other hand, if the adhesive layer is insufficiently tacky, at the time of tire production, there is the problem that the thermoplastic elastomer layer will end up peeling off from the tire carcass thereby making formation impossible. See U.S. Publication No. 2007-0031661A1. Further, the thermoplastic elastomer requires an extruder processing melt temperature in excess of the vulcanization temperature of the adhesive at which the adhesive is later cured with the rubber in the tire to adhere the innerliner to the tire carcass; there is the problem of the occurrence of scorching or other excessive thermal contamination from the thermoplastic elastomer in the process of co-extrusion with the thermoplastic elastomer composition at a high temperature, resulting in crosslinking, gel formation and/or degradation in the adhesive layer, reducing its efficacy in the tire.
There is a continuing need to improve the properties of dynamically vulcanized thermoplastic barrier films based on nylon and elastomers, particularly where the use of carbon black in the elastomer phase is avoided.
The limitations of the prior art approaches to achieving improved performance of the desirable compositions comprising a small particle size rubber domain dispersed in a thermoplastic matrix, the composition exhibiting improved gas or fluid barrier properties and desirable levels of strength and durability suitable for use in tires and hose applications suggest that there is a continuing need for improvements, as have been accomplished by use of the processes and compositions of the present invention.